1. Field of the Invention
The present invention relates to a magnetic resonance imaging (abbreviated as "MRI" hereinafter) apparatus for obtaining tomograms of desired sections of an object to be examined by utilizing nuclear magnetic resonance (abbreviated as "NMR" hereinafter). In particular, it relates to a method for magnetic resonance angiography (referred to as "MRA" hereinafter), which is not likely to be affected by variation in blood flow rate caused by cardiac beat when imaging blood flow in a vascular system, and an apparatus therefor.
2. Related Art
In the MRI, a radio frequency magnetic field is applied to an object to be examined to excite atomic nucleus spins of atoms constituting living tissues of the object, and NMR signals elicited by the spins are acquired to form an image from the spatial distribution of the spins or spectra. When applying the radio frequency magnetic field, gradient magnetic fields are simultaneously applied to impart locational information. The atomic nucleus spins measured in MRI are usually atomic nucleus spins of hydrogen atoms. While the spins of atoms constituting tissues are static, the spins of atoms present in blood flows move. Based on this fact, various blood flow imaging techniques (MRA) using an MRI apparatus have been developed.
Most of such MRA techniques fall in three categories, i.e., the time-of-flight (TOF) method which utilizes the effect of blood inflow into slice planes, the phase-sensitive (PS) method utilizing subtraction of data acquired in the presence and absence of phase diffusion and the phase-contrast (PC) method utilizing subtraction of data acquired in two different conditions where the polarity of phase diffusion caused by blood flow is opposite.
The latter two utilize the fact that moving spins such as in blood flow experience phase modulation relative to their flow rate when the gradient magnetic fields are applied. In the PS method, blood flow imaging is performed by carrying out two kinds of sequences, one for applying gradient magnetic fields so that the phase of moving spins does not coincide with the phase of stationary spins at the timing of signal acquisition (dephase sequence) and another for applying gradient magnetic fields so that the phase of moving spins coincides with the phase of static spins at the timing of signal acquisition (rephase sequence), and using the difference between the data obtained by these sequences. The PC method utilizes a sequence for applying gradient magnetic fields with different polarities (called flow encode gradient magnetic fields) in the direction of blood flow to collect data and uses the difference between the acquired data. In this method, by generating six sequences, data for three axes can be obtained. A technique for obtaining data for three axes by generating four sequences has also been suggested for this method.
It is well-known that the blood flow rate of the vascular system, in particular, the arterial system, changes with the cardiac cycle. This variation in blood flow rate may cause variation in the intensity of the blood flow signal in the MRA measurement. In the PC method and the PS method in particular, the phase of moving spins changes with variation in the blood flow rate, and this leads to changes in the intensity of the differential signals. However, in the conventional methods, the signal acquisition is performed irrespective of the cardiac cycle. Therefore, depending on the cardiac phase when the echo signals are collected and the order of filling the components of the measurement space (space where data are arranged, also called k-space) with acquired data, contrast of the image obtained may change and artifacts may be produced.
Specifically, the phase of the spins is locational information obtained by applying the gradient magnetic fields. It may vary depending on the location in real space (in proportion to the distance from the magnetic field center) and should be inherently constant for each phase encode step. However, the spins of atoms constituting blood flow experience a rotation cyclically varying with the beating of the heart, and the locational information imparted to the blood flow signals therefore varies every encode step. As a result, blood flow signals cannot be formed into an image of one position but appear as artifacts drifting along the phase direction. This problem becomes pronounced particularly in the PC method and the PS method, because the flow encode pulse or dephase pulse for imparting a phase rotation in proportion to the flow rate to the constant flow is applied.
It is known that electrocardiography (ECG) gated measurement is effective for preventing the change of signal strength or artifacts caused by the heart beat. FIG. 10 shows a sequence for the PS method utilizing the ECG-gated measurement. In this sequence, a rephase sequence is repeated within one cardiac cycle and a dephase sequence is repeated within the next cardiac cycle to acquire one set of data for one phase encode step within two cardiac cycles. In this ECG-gated measurement, data for one image are always collected at a fixed cardiac phase and the k-space is filled with data of the same cardiac phase. Accordingly, in the measurement by the sequence shown in FIG. 10, image for each cardiac phase (L images for the first cardiac phase to the Lth cardiac phase) can be obtained and the images are free from nonuniformity of image contrast and artifacts.
However, in this method, measurement for one phase encode step is performed over two cardiac cycles. Therefore, in order to perform J number of encode steps necessary for constituting one image, it is necessary to repeat a sequence over cardiac cycles in a number of twice the number of the phase encode steps (2J). Therefore, supposing that one cardiac cycle is about 1 second and J is 256, one measurement takes about 8.5 minutes (=256.times.2/60). In the PC method, in addition, because one set of data for one phase encoding are obtained by a combination of four or six different sequences, the measurement time is further prolonged. Therefore, it may be practically impossible to utilize the ECG-gated measurement for three-dimensional imaging and hence its application may be limited to two-dimensional imaging.
Accordingly, the object of the present invention is to provide a method for MRA, which can provide stable images while obviating prolonged measurement time.